Conventionally, metal particles, alloy particles, metal oxide particles, etc., supported on carrier particles have been used as catalysts for various uses including deodorants, antibacterial agents, automobile exhaust gas purifiers, fuel cells, and Nox reducers. Materials used for such carrier particles include carbon and metal oxides such as titanium oxide, zirconium oxide, iron oxide, nickel oxide, and cobalt oxide. In particular, catalysts comprising electrically conductive carbon particles as carrier can serve effectively as catalyst for fuel cell electrodes.
Among others, materials comprising a carbon carrier that supports platinum-ruthenium alloy particles and those comprising a carbon carrier that supports fine metallic platinum particles along with specific metal oxide particles, such as molybdenum oxide and cerium dioxide, as promoter have been known to serve effectively as catalyst for electrodes. Patent document 1, for instance, describes that agglomeration of platinum particles can be reduced by depositing particles of a corrosion resistant oxide, such as cerium dioxide and zirconium oxide, on platinum particles followed by depositing the platinum particles on a carbon carrier. Patent documents 2 and 3 propose an electrode catalyst that is produced by depositing particles of noble metal, such as platinum, on the surface of perovskite type titanium oxide particles and applying a paste of said noble metal-supporting oxide over a carbon membrane and describe that the perovskite type titanium oxide works as promoter to improve the catalytic ability.
On the other hand, some perovskite type composite metal oxides that are transition metal oxides with specific structures are known to be able to decompose NOx, and Patent document 4 proposes NOx contact catalysts comprising a carrier supporting such oxides. Patent document 5 describes that catalysts produced by depositing a noble metal, such as Pt, Pd and Rh, on such a perovskite type Fe oxide carrier show high catalytic ability at very high temperatures above 500° C. Patent document 6 describes, furthermore, that a catalyst that works effectively not only at high temperatures but also at low temperatures and has improved resistance to sulfur poisoning can be produced by replacing part of the Fe sites in the perovskite type Fe oxide (represented by the general formula AFeO3) with noble metal atoms such as Pt, Pd, and Rh.
Some perovskite type composite metal oxides comprising transition metal elements such as iron, cobalt, and nickel have already been put to practical use as catalyst for air electrodes in solid oxide fuel cells (SOFC). Solid oxide fuel cells are used in a high temperature environment at about 800° C. or more, but it has been known that at such high temperatures, the transition metal elements contained can work by themselves as oxygen-degradable catalyst.
Patent document 7 described, furthermore, that sintering of the platinum particles on the carrier can be prevented and the usage of costly platinum particles can be decreased if alumina, silica, manganese oxide, iron oxide, cobalt oxide, or other metal oxide particles coexist with the supported platinum particles on the carbon particles.
Common processes available to deposit various metal oxides on the surface of a carrier include the following:
(1) To allow a carrier to adsorb metal colloid particles,
(2) To disperse carrier particles in an aqueous metal salt solution, and use an alkaline chemical agent to allow metal hydroxide to precipitate on the carrier surface
(3) To use fine particles to prepare a fine particle dispersion liquid, and then allow the fine particles to be fixed on the surface of a carrier.
Known methods that use such liquid phase processes are proposed in Patent documents 8 and 9. In Patent document 8, platinum-supporting carbon particles are dispersed in a mixed solution of appropriate metal salts, and the hydroxides of said metals are precipitated on the carbon particles using an alkaline chemical agent, followed by heating the solution in a reducing environment at 1000° C. or above to allow the carbon particles to support fine alloy particles (fine particles of four metal alloy of platinum, molybdenum, nickel, and iron). The document specifies that the fine alloy particles should have a size of about 3 nm or more.
In the process to produce carbon particles supporting vanadium pentoxide proposed in Patent document 9, an organic solvent is added to the organic vanadium solution for solvation to produce organic complexes, which are then adsorbed on the carbon particles. In this case, the vanadium pentoxide supported on the carbon particles is in an amorphous state.
To deposit a perovskite type oxide on a carrier surface, the carrier may be coated with an aqueous solution containing its metal salt, dried and heat-treated at a high temperature to cause its precipitation on the carrier surface. For instance, a process to produce a carrier supporting perovskite type iron fine oxide particles is proposed in Patent document 10, which consists of synthesizing perovskite type iron oxide particles having Pd contained in their crystal lattice, using them to produce slurry, coating the carrier with the slurry, and heat-treating it. For this process, the perovskite type iron oxide particles synthesized first had a submicronic size, and the carrier had a sufficient surface area for coating with the slurry.
Besides, Patent document 11 describes a method using microwave plasma treatment to deposit metal oxide particles on carbon-based material. In the processes given as example, titanium oxide, nickel oxide, and cobalt oxide are deposited on carbon, and the document describes that the method can be applied to perovskite type composite metal oxides. With this method, it is possible to allow a carbon-based carrier to support a metal oxide that cannot be deposited easily on a carbon material because it requires a high oxidation temperature where the carbon material starts to burn, although this method requires special equipment to carry out the plasma treatment.
[Patent document 1] Japanese Unexamined Patent Publication (Kokai) No. 2004-363056
[Patent document 2] Japanese Unexamined Patent Publication (Kokai) No. 2005-50759
[Patent document 3] Japanese Unexamined Patent Publication (Kokai) No. 2005-50760
[Patent document 4] Japanese Unexamined Patent Publication (Kokai) No. Hei 5-261289
[Patent document 5] Japanese Unexamined Patent Publication (Kokai) No. 2001-269578
[Patent document 6] Japanese Unexamined Patent Publication (Kokai) No. 2004-321986
[Patent document 7] Japanese Unexamined Patent Publication (Kokai) No. 2005-270873
[Patent document 8] Japanese Unexamined Patent Publication (Kokai) No. Hei 5-217586
[Patent document 9] Japanese Unexamined Patent Publication (Kokai) No. 2000-36303
[Patent document 10] Japanese Unexamined Patent Publication (Kokai) No. 2004-41866
[Patent document 11] Japanese Unexamined Patent Publication (Kokai) No. Hei 11-28357